A drilling fluid is a specially designed fluid that is circulated through an oil or gas wellbore as the wellbore is drilled to facilitate the drilling operation. A need exists for an improved additive for modifying and controlling the suspension properties of drilling fluids that would be efficient, easily handled, and readily dispersible in a broad range of drilling muds, and usable under a broad range of conditions.
Drilling fluids or muds are typically classified according to their base fluid or continuous phase, as water base and oil base fluids. Drilling fluids may contain a mixture of base fluids, and are typically classified by the predominating or continuous base fluid, with the fluid present in lesser quantities becoming the internal or emulsified phase. The use of oil and invert oil-based drilling fluids or muds in oil exploration is increasing rapidly owing to the more demanding requirements encountered in drilling deep and/or non-vertical and deviated wells. Compared with the longer-established water-based drilling muds, oil and invert oil-based drilling fluids possess a number of advantages, including reduced interaction with earth formations, and improved lubricity. The drilling fluids and methods of the present disclosure are particularly useful in invert emulsion systems.
Oil based invert emulsion drilling fluids are generally used throughout the world and consist of a three-phase system: oil, internal hygroscopic phase and fine particulate solids. The internal hygroscopic phase can be an aqueous phase typically a brine or it can be an organic compound like glycerol or polyglycerol either dissolved in water or neat or a combination thereof. The addition of brine reduces the overall price of the fluid, reduces the risk of combustion of the oil, builds an emulsion structure that provides suspension and improved viscosity to the fluid, provides improved shale stability by driving water out of the shale and into the fluid via osmosis, and improves the water acceptance of the mud. The brine of choice is commonly an aqueous solution of an inorganic salt, such as sodium chloride or calcium chloride.
Drilling fluids or drilling muds are pumped under pressure down through a long string of drill pipe, then through the center of the drilling bit at the hole bottom, then back up through the annulus between the outside of the string of drill pipe and up the borehole wall to the surface.
Drilling fluids provide a number of interrelated functions to satisfy the requirements of the oil industry for a commercial drilling fluid, which may be grouped as follows. (1) The drilling fluid must suspend and transport solid particles, e.g. drill cuttings, to the surface for screening out and disposal. (2) The drilling fluid must build a filter cake that can prevent the loss of downhole pressure and fluid loss to the formation, including when traversing an interval of porous formation material. (3) The drilling fluid must keep suspended an additive weighting agent (to increase specific gravity of the mud), so that uniform mud weight is maintain throughout the column of drilling fluid in the well, especially when encountering pressurized pockets of combustible gas, which otherwise would tend to reduce downhole pressure, as well as creating a “blowout” in which the fluid and even the drill stem are violently ejected from the well, with resulting catastrophic damages, such as fires. (4) The drilling fluid must constantly lubricate the drill bit so as to promote drilling efficiency and retard bit wear. (5) The drilling fluid should maintain sufficient hydrostatic pressure to manage the wellbore pressure for improved wellbore stability.
It should be noted that a drilling fluid must perform its various functions not only when the drill bit is actively encountering the bottom of the borehole, but also at times when the drill stem is inactive, or is being removed or re-inserted for some purpose. In particular, cuttings must be held in suspension in the event of shut-downs in drilling.
An ideal drilling fluid is a thixotropic system. That is, the drilling fluid (1) will exhibit low viscosity when sheared, such as during agitation or circulation (as by pumping or otherwise) but, (2) when the shearing action is halted, the fluid must gel to hold the cuttings in place, and it must become gelled relatively rapidly, reaching a sufficient gel strength before suspended materials fall any significant distance, and (3) this behavior should be almost completely reversible. In addition, even when it is a free-flowing liquid, the fluid must retain a sufficiently high viscosity to carry all unwanted particulate matter from the bottom of the hole to the surface. Moreover, upon long-term interruption of circulation, such as when drilling fluid has been ejected from the borehole into a quiescent holding vessel or pond, the gel structure should remain intact to allow the weighting agent particles to remain suspended and maintain a uniform distribution throughout the fluid.
During the drilling process, some of the drilled solids erode and become finer particles that cannot be removed via solids control equipment (at the surface) due to their small size. As drilling progresses these particles become finer to colloidal size and increase in their concentration in the drilling fluid. These colloidal fines and their concentration can negatively impact the performance of the invert emulsion fluid (IEF). The performance of the IEF can be gauged by the American Petroleum Institute (API) fluid loss test, rheology and monitoring for emulsion stability. These colloidal fines can either cause an increase in the API fluid loss, increase the overall rheology of the fluids or at the very end may even destabilize the IEF. This increase in the overall rheology of the fluid can be determined by an increase in Fann 35 rheometer readings from 600 rotations per minute (rpm) to 3 rpm. An increase in overall rheology of the fluid increases the equivalent circulating density (ECD) of the fluid. The ECD is the effective density of the circulating fluid in the wellbore resulting from the sum of the hydrostatic pressure imposed by the static fluid column and the friction pressure. API STD 65—Part 2, Isolating Potential Flow Zones During Well Construction, Upstream Segment, Second Edition, December 2010. Global Standards. More specifically, an increase in the rheology increases the frictional pressure component in the ECD equation, and thereby, the ECD of the fluid. The ECD is an important parameter in avoiding kicks and losses of the fluid, particularly in wells that have a narrow window between the fracture gradient and pore-pressure gradient.
When the colloidal fines in the IEF increase, the fluid may be treated in one or more of the following ways, for example, by: (i) adding a thinner to lower the rheology; (ii) adding wetting agents to maintain the water wettability of the colloidal fines; (iii) adding fresh IEF to dilute the overall drilling fluid to reduce the overall concentration of the low gravity solids (LGS); (iv) performing high speed centrifuge operations to remove the finer solids; and/or (v) adding other additives to maintain a workable fluid. However, at times these treatments may not work and the whole of the used IEF has to be discarded and fresh IEF made.
It is therefore advantageous to have emulsifiers or additives as part of the IEF that reduce the maintenance/treatments of these fluids by maintaining controlled (low) rheology profile and API fluid loss performance of the IEF. This improvement will in turn increase the overall working longevity of the IEF. Thus, there is a need for a cost effective drilling fluid that can perform all the above mentioned functions. The present disclosure provides a drilling fluid with an IEF composition having low titer low titer fatty acid material (or low titer rosin acid containing fatty acid material) based specialty emulsifier which delivers a low rheology IEF even in the presence of a high concentration of low gravity solids (LGS). A low rheology in turn leads to lower induced fluid losses in the drilling fluid formation when drilling oil and gas wells.